libcwd: Debug output fixes.

Print 'Connection closed' debug message of libcurl before the
'Connection closed' Dout to dc::curlio.
Add MultiHandle::upload_finished_poll

After fixing libcurl, it closes connections a lot less,
reuses just a single connection for pipelining - and doesn't call the
socket callback as often anymore. As a result, the current 'upload
finished' detection stopped working: libcurl uses a quick poll(2) to
decide if it can write to a socket (and it always can) and then send GET
requests to the server internally. Absolutely NOTHING can be seen about
that from the outside, except that the pretransfer time on those
requests changed... Hence, we now regularly poll all added requests to
see if this variable was set and use that as our 'uploaded finished'
detection.
Accessors that I keep needing for debugging.
Just because...
libcwd: specialized curl debug code.

This needs a specially hacked libcurl that abuses the socket callback
with an 'action' value of 1234567 to inform the viewer about its
internal connectdata structs, which subsequently are used to display
live graphical feedback in the HTTP console concerning possible
filedescriptor problems. File descriptors of active connections are
shown, followed by 'wr' when they need to be watched for writing or
reading respectively and '--' (or 'w-' or '-r') when not. While the
color of each ('w' and 'r') is red when the viewer isn't watching that
socket for writability and/or readablity.

Note that what followed is that libcurl *seldom* requires testing for
writability: it does that internally with a poll and just writes the
data without asking the application for it.